Many types of hydraulic hoists have been developed over the years which are particularly useful as underbody hoists for raising and lowering a dump bed, such as the dump bed of a heavy duty dump truck or trailer. Generally speaking, these hoists employ hydraulic cylinders which, when activated, pivot the dump bed about its rear pivots connected to the frame of the truck chassis.
In a typical hydraulic truck hoist, a hydraulic cylinder lies lengthwise between the framerails of the vehicle and underneath the truck bed. As the hoist's cylinder extends, one end pushes against the underside of the dump bed while the other end of the cylinder, being pivotally attached to the frame members (usually cross members) of the vehicle, pivots to allow the dump bed to raise. At the beginning of the lifting process, the cylinder is nearly parallel to the dump bed and thus has very little initial leverage. Most of the force of the cylinder at this point in the lift process is directed towards the rear hinge that connects the dump bed to the truck frame. Due to this inefficiency, large cylinders are required to provide the substantial forces necessary to achieve this initial lift.
Many types of hydraulic hoists have been developed to address this initial lift problem. In one example, telescopic cylinders are mounted farther forward on the truck frame. This provides the cylinder with greater initial leverage. However, the cylinder in such devices usually must have a rather long stroke to achieve a sufficient dump angle.
Other attempts to improve these hoists have included the use of a simple lever arm to provide the lifting motion in an attempt to obtain a more efficient lift angle. Examples include those hoists disclosed in U.S. Pat. No. 4,052,105. Other designs include those disclosed in U.S. Pat. Nos. 2,509,911; 2,603,518; and 4,762,370. These patents disclose hoist designs where a lever provides the initial lift of the truck bed, and a cylinder completes the lifting process.
Generally speaking, truck hoist designs known prior to the subject invention have one or more drawbacks associated with their use. For the most part, they often require large and expensive cylinders. Many are difficult to install or manufacture. Other designs are subject to large stresses, are potentially unstable when elevated, or do not achieve superior initial lift. Stability problems have been known to occur, for example, when wet loads are carried and the entire load is not eliminated during the dumping operation, leaving, for example, some of the load in the most elevated part of the dump bed.
In view of the above, it is apparent that there exists a need in the art for a hoist which can efficiently raise and lower a truck bed and which is not subject to the above drawbacks. It is a purpose of this invention to fulfill this need in the art, as well as other needs which will become apparent to the skilled artisan once given the following disclosure.